Power converter

ABSTRACT

In a power conversion apparatus having a plurality of AC power supplies, even if an abnormality occurs in any one of the AC power supplies and loads that are respectively connected to two capacitors are not balanced, voltages of the two capacitors are balanced.  
     In the power conversion apparatus of the present invention, in the case where an abnormality occurs in a first or second AC power supply, a first reactor is connected to a second reactor in series, a battery supplies energy to the two capacitors, and a current flowing to the first reactor is controlled by the series body of a first switching means. In this manner, there is controlled a voltage difference between the two capacitors.

TECHNICAL FIELD

[0001] The present invention relates to a power conversion apparatussuch as an uninterruptible power supply that is capable of supplyingpower to a load even if an abnormality, such as a power failure or amomentary voltage drop, occurs in a system power supply. Moreparticularly, the present invention relates to an apparatus having afunction of coping with unbalanced loads.

BACKGROUND ART

[0002] A conventional AC/DC conversion apparatus disclosed in JP PatentNo. 2765372 is shown in FIG. 24. In this drawing, reference numeral 100denotes an AC power supply, numeral 101 a switch means, numeral 102 areactor, numerals 103 and 104 transistors, numerals 105 and 106 diodes,numerals 107 and 108 capacitors, numerals 109 and 110 resistorsfunctioning as DC loads, and numeral 111 a battery.

[0003] In the apparatus constructed in this manner, in the case wherethe AC power supply 100 operates normally, the switch means 101 isconnected to a contact point A and the transistors 103 and 104 arealternately turned on/off. As a result, a current of the reactor 102 iscontrolled so that the power factor of a current flowing to the AC powersupply 100 becomes one, and the capacitors 107 and 108 are charged.Also, the charged energy is supplied to each of the resistors 109 and110.

[0004] Also, in the case where an abnormality, such as a power failure,occurs in the AC power supply 100, energy is supplied from the battery111 to the resistors 109 and 110. During this operation, in the casewhere the resistors 109 and 110 have the same resistance value, that is,in the case where these resistors consume the same amount of power, theamount of energy supplied from the capacitor 107 to the resistor 109becomes the same as that supplied from the capacitor 108 to the resistor110. As a result, the voltages of the capacitors 107 and 108 becomeequal to each other at all times. That is, the voltage values of thecapacitors 107 and 108 are balanced by the energy supplied from thebattery 111.

[0005] However, in the case of unbalanced loads where values of theresistors 107 and 108 differ from each other, for instance, voltagesvalues of the capacitors 107 and 108 are unbalanced. This is becauseonly the battery 111 charges the two capacitors 107 and 108 and itbecomes impossible to control the potential at an interconnection pointC between the capacitors 107 and 108.

[0006] Even if the AC power supply operates normally, in the case ofsuch unbalanced loads, there occurs such unbalance. However, forinstance, JP Laid-Open No. 2000-278954 discloses a technique ofeliminating unbalance by changing the ratio between on/off times duringthe switching of the transistors 103 and 104 using an unillustratedcontrol circuit.

[0007] By the way, if an abnormality like a power failure occurs in theAC power supply 100 and the voltages of the capacitors 107 and 108 areunbalanced, there occurs a problem that desired voltages are not appliedto the loads 109 and 110. That is, in the case of unbalanced loads wherethe value of the resistor 110 is smaller than the value of the resistor109, for instance, the voltage of the capacitor 108 ultimately becomeszero and the voltage of the capacitor 107 becomes VB.

[0008] In view of this problem, with the conventional techniquedisclosed in the above-mentioned publication, in the case where anabnormality like a power failure occurs in the AC power supply 100, theswitch means 101 is switched to a contact point B. Also, to stabilizethe potential at the interconnection point C, the current of the reactor102 is controlled by performing the switching of the transistors 103 and104 using an unillustrated control circuit. As a result of thisoperation, the voltages of the two capacitors 107 and 108 become equalto each other at all times even in the case of the unbalanced loads.Also, power supply to the loads 109 and 110 is performed with stability.

[0009] By the way, in the case of a single-phase three-wire system ACpower supply, conversion blocks 114 and 115, which each includes areactor, a transistor, and a diode like in FIG. 24, are connected to ACpower supplies 112 and 113, as shown in FIG. 25. In this drawing,reference numerals 116 and 117 denote capacitors, numerals 120 and 121resistances, and numeral 124 a battery.

[0010] In the case of such a single-phase three-wire system, if there ismade an attempt to use the conventional technique disclosed in JP PatentNo. 2765372 described above that is also capable of coping withunbalanced loads when an abnormality occurs in an AC power supply, theremay be conceived a method with which control is performed so that thevoltage values of the capacitors 116 and 117 are balanced by switchingcontact points of the switch means 125 and 126 as shown in FIG. 26.However, with the construction shown in FIG. 26, the voltages VC1 andVC2 of the capacitors 116 and 117 are applied to the reactors within theconversion blocks 114 and 115 as they are, so that there occurs aproblem that ripple currents flowing to the reactors become large. As aresult, the efficiency of AC/DC conversion is lowered or noises from thereactors are increased.

[0011] Also, as shown in FIG. 27, there may be a case where a filtercapacitor 130 is connected to remove a ripple current that occurs in thereactor 102 due to the switching of the transistors 103 and 104. In thiscase, as shown in FIG. 28, when it is detected that an abnormalityoccurs in the AC power supply 100 (T(fault)) and the switch means 101 isswitched from “A” to “B”, a voltage remains in the filter capacitor 130.Consequently, a steep current that leads to the discharging of thevoltage of the filter capacitor 130 is generated concurrently with theswitching of the switch means 101. Also, this steep current flows to theswitch means 101, so that there may occur a problem that the switchmeans 101 is damaged by an excess current. FIG. 28(a) shows a voltagewaveform of the AC power supply 100, FIG. 28(b) shows a voltage waveformof the filter capacitor 130, and FIG. 28(c) shows a current waveform ofthe filter capacitor 130.

[0012] The present invention has been made to solve the problemsdescribed above, and a first object of the present invention is toprovide a power conversion apparatus having a plurality of AC powersupplies, wherein even if an abnormality occurs in at least one of theAC power supplies and loads that are respectively connected to twocapacitors are not balanced, the power conversion apparatus is capableof balancing voltages of the two capacitors and is also capable ofreducing losses and noises by decreasing a ripple current flowing to areactor during the switching of the transistors 103 and 104.

[0013] Also, in the case where a filter capacitor is connected, a secondobject of the present invention is to provide a power conversionapparatus that is capable of preventing a situation where a switch meansis damaged because electric charges of the filter capacitor aredischarged and a steep current flows to the switch means when the switchmeans is turned on.

[0014] Further, in the case where a filter capacitor is connected, athird object of the present invention is to provide a power conversionapparatus that is capable of reducing losses by suppressing unnecessaryresonance that occurs between the filter capacitor and a reactor when aswitch means is turned on.

DISCLOSURE OF THE INVENTION

[0015] According to the present invention, a first power conversionapparatus is provided with: a first AC/DC conversion means constructedby connecting a first AC power supply, a first reactor, and a seriesbody of a first switching means in series; a second AC/DC conversionmeans constructed by connecting a second AC power supply, a secondreactor, and a series body of a second switching means in series; twocapacitors connected in series, an interconnection point of the twocapacitors being connected to one end of each of the two AC powersupplies, and the two capacitors receiving energy supplied by DCvoltages obtained by the first and second AC/DC conversion means; loadsthat are respectively connected to the two capacitors; and a batteryconnected to the two capacitors that are connected in series,

[0016] the power conversion apparatus comprising:

[0017] a first switch means that is connected between the first AC powersupply and the first reactor, connects the first reactor to the first ACpower supply if the first and second AC power supplies operate normally,and connects the first reactor to a connection point between the secondreactor and the series body of the second switching means if one of thefirst and second AC power supplies operates abnormally;

[0018] a second switch means that is connected between the second ACpower supply and the second reactor, connects the second reactor to thesecond AC power supply if the first and second AC power supplies operatenormally, and connects the first reactor and the second reactor inseries by connecting the second reactor to an interconnection pointbetween the capacitors if one of the first and second AC power suppliesoperates abnormally; and

[0019] a control apparatus that controls a voltage difference betweenthe two capacitors, wherein:

[0020] if the first and second AC power supplies operate normally, thecontrol apparatus controls a current flowing to the first reactor usingthe series body of the first switching means to perform AC/DC conversionand controls a current flowing to the second reactor using the seriesbody of the second switching means to perform AC/DC conversion; and

[0021] if one of the first and second AC power supplies operatesabnormally, the control apparatus has the battery supply energy to thetwo capacitors and controls the current flowing to the first reactorusing the series body of the first switching means.

[0022] With this construction, in the power conversion apparatus havingthe plurality of AC power supplies, when at least one of the AC powersupplies operates abnormally, even in the case where loads that arerespectively connected to two capacitors are unbalanced, it is possibleto balance voltages of these capacitors. Also, during this operation,there is obtained an effect that losses and noises are reduced byreducing ripple currents flowing to the reactors due to theopening/closing of the first switching means.

[0023] According to the present invention, a second power conversionapparatus is provided with: a first AC/DC conversion means constructedby connecting a first AC power supply, a first reactor, and a seriesbody of a first switching means in series; a second AC/DC conversionmeans constructed by connecting a second AC power supply, a secondreactor, and a series body of a second switching means in series; twocapacitors connected in series, an interconnection point of the twocapacitors being connected to one end of each of the two AC powersupplies, and the two capacitors receiving energy supplied by DCvoltages obtained by the first and second AC/DC conversion means; loadsthat are respectively connected to the two capacitors; and a batteryconnected to the two capacitors that are connected in series,

[0024] the power conversion apparatus comprising:

[0025] a first switch means that is connected between the first AC powersupply and the first reactor;

[0026] a second switch means that is connected between the second ACpower supply and the second reactor;

[0027] a third switch means that is connected between a connection pointbetween the first switch means and the first reactor, and a connectionpoint between the second reactor and the series body of the secondswitching means;

[0028] a fourth switch means that is connected between a connectionpoint between the second switch means and the second reactor, and aninterconnection point between the capacitors;

[0029] a series body of a fifth switch means and a first filtercapacitor that is connected between a connection point between the thirdswitch means and the first reactor, and an interconnection point betweenthe capacitors;

[0030] a second filter capacitor that is connected between a connectionpoint between the fourth switch means and the second reactor, and aninterconnection point between the capacitors; and

[0031] a control apparatus that controls a voltage difference betweenthe two capacitors, wherein:

[0032] if the first and second AC power supplies operate normally, thecontrol apparatus turns on the first switch means, the second switchmeans, and the fifth switch means and turns off the third switch meansand the fourth switch means, so that a current flowing to the firstreactor is controlled using the series body of the first switching meansto perform AC/DC conversion, a high-frequency current flowing to thefirst reactor is absorbed using the first filter capacitor, a currentflowing to the second reactor is controlled using the series body of thesecond switching means to perform AC/DC conversion, and a high-frequencycurrent flowing to the second reactor is absorbed using the secondfilter capacitor; and

[0033] if one of the first and second AC power supplies operateabnormally, the control apparatus turns off the first switch means andthe second switch means, sets a voltage of the second filter capacitorto approximately zero through switching of the series body of the secondswitching means, sets a current of the first reactor to approximatelyzero through switching of the series body of the first switching means,turns on the third switch means and the fourth switch means, turns offthe fifth switch means, connects the first reactor to a connection pointbetween the second reactor and the series body of the second switchingmeans, and connects the first reactor to the second reactor in series,so that energy is supplied to the two capacitors using the battery and acurrent flowing to the first reactor is controlled using the series bodyof the first switching means.

[0034] With this construction, in the power conversion apparatus havingthe plurality of AC power supplies, when at least one of the AC powersupplies operates abnormally, even in the case where loads that arerespectively connected to two capacitors are unbalanced, it is possibleto balance voltages of these capacitors. Also, during this operation,there is obtained an effect that it is possible to reduce losses andnoises by reducing ripple currents flowing to reactors due to theopening/closing of the first switching means. Also, there iscircumvented a situation where electric charges of the second filtercapacitor are discharged when the fourth switch means is turned on andtherefore a steep current flows to the fourth switch means and thefourth switch means is damaged. Also, the fifth switch means is turnedoff by setting the current of the first reactor to zero, so that thereis circumvented a problem that the fifth switch means is damaged byenergy accumulated in the first reactor when the fifth switch means isturned off.

[0035] According to the present invention, a third power conversionapparatus is provided with: a first AC/DC conversion means constructedby connecting a first AC power supply, a first reactor, and a seriesbody of a first switching means in series; a second AC/DC conversionmeans constructed by connecting a second AC power supply, a secondreactor, and a series body of a second switching means in series; twocapacitors connected in series, an interconnection point of the twocapacitors being connected to one end of each of the two AC powersupplies, and the two capacitors receiving energy supplied by DCvoltages obtained by the first and second AC/DC conversion means; loadsthat are respectively connected to the two capacitors; and a batteryconnected to the two capacitors that are connected in series,

[0036] the power conversion apparatus comprising:

[0037] a first switch means that is connected between the first AC powersupply and the first reactor;

[0038] a second switch means that is connected between the second ACpower supply and the second reactor;

[0039] a third switch means that is connected between a connection pointbetween the first switch means and the first reactor, and a connectionpoint between the second reactor and the series body of the secondswitching means;

[0040] a fourth switch means that is connected between a connectionpoint between the second switch means and the second reactor, and aninterconnection point between the capacitors;

[0041] a first filter capacitor that is connected between a connectionpoint between the third switch means and the first reactor, and aninterconnection point between the capacitors;

[0042] a second filter capacitor that is connected between a connectionpoint between the fourth switch means and the second reactor, and aninterconnection point between the capacitors; and

[0043] a control apparatus that controls a voltage difference betweenthe two capacitors, wherein:

[0044] if the first and second AC power supplies operate normally, thecontrol apparatus turns on the first switch means and the second switchmeans and turns off the third switch means and the fourth switch means,so that a current flowing to the first reactor is controlled using theseries body of the first switching means to perform AC/DC conversion, ahigh-frequency current flowing to the first reactor is absorbed usingthe first filter capacitor, a current flowing to the second reactor iscontrolled using the series body of the second switching means toperform AC/DC conversion, and a high-frequency current flowing to thesecond reactor is absorbed using the second filter capacitor; and

[0045] if one of the first and second AC power supplies operateabnormally, the control apparatus turns off the first switch means andthe second switch means, sets a voltage of the first filter capacitor toapproximately zero through switching of the series body of the firstswitching means, sets a voltage of the second filter capacitor toapproximately zero through switching of the series body of the secondswitching means, turns on the third switch means and the fourth switchmeans, connects the first reactor to a connection point between thesecond reactor and the series body of the second switching means, andconnects the first reactor to a parallel connection body of the secondreactor and the first filter capacitor in series, so that energy issupplied to the two capacitors using the battery and a current flowingto the first reactor is controlled using the series body of the firstswitching means.

[0046] With this construction, in the power conversion apparatus havingthe plurality of AC power supplies, when at least one of the AC powersupplies operates abnormally, even in the case where loads that arerespectively connected to two capacitors are not balanced, it ispossible to balance voltages of these capacitors. Also, during thisoperation, there is obtained an effect that it becomes possible toreduce losses and noises by reducing ripple currents flowing to reactorsdue to the opening/closing of the first switching means. Also, there iscircumvented a situation where electric charges of the second filtercapacitor are discharged when the fourth switch means is turned on andtherefore a steep current flows to the fourth switch means and thefourth switch means is damaged. Further, it is also possible to suppressunnecessary resonance between the first filter capacitor and the secondreactor when the third switch means is turned on, which makes itpossible to reduce losses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a circuit diagram showing a circuit construction of apower conversion apparatus according to a first embodiment;

[0048]FIG. 2 is an equivalent circuit diagram illustrating an operationperformed when an abnormality occurs in an AC power supply in the powerconversion apparatus according to the first embodiment;

[0049]FIGS. 3 and 4 are explanatory drawings illustrating an operationperformed when an abnormality occurs in an AC power supply in the powerconversion apparatus according to the first embodiment;

[0050]FIG. 5 is an explanatory drawing illustrating a current waveformin the equivalent circuit shown in FIG. 2 according to the firstembodiment and a gate signal waveform of a switch means;

[0051]FIGS. 6 and 7 are explanatory drawings illustrating anotheroperation performed when an abnormality occurs in the AC power supply inthe power conversion apparatus according to the first embodiment;

[0052]FIG. 8 is an explanatory drawing illustrating another currentwaveform in the equivalent circuit shown in FIG. 2 according to thefirst embodiment and a gate signal waveform of the switch means;

[0053]FIG. 9 is a circuit diagram illustrating a control circuit thatbalances voltages of two capacitors according to the first embodiment;

[0054]FIG. 10 is a circuit diagram illustrating a circuit constructionof another power conversion apparatus according to the first embodiment;

[0055]FIG. 11 is a circuit diagram illustrating a circuit constructionof a power conversion apparatus according to a second embodiment;

[0056]FIG. 12 is an equivalent circuit diagram illustrating an operationperformed when an abnormality occurs in an AC power supply in the powerconversion apparatus according to the second embodiment;

[0057]FIG. 13 is an equivalent circuit diagram illustrating a statebefore third and fourth switch means according to the second embodimentare turned on;

[0058]FIG. 14 is a circuit diagram illustrating a control circuit thatcontrols a reactor current in the equivalent circuit in FIG. 13according to the second embodiment;

[0059]FIG. 15 is a drawing illustrating a time chart of control of eachswitch means according to the second embodiment;

[0060]FIG. 16 is a drawing illustrating another time chart of thecontrol of each switch means according to the second embodiment;

[0061]FIG. 17 is a flowchart illustrating an example of the control ofeach switch means according to the second embodiment;

[0062]FIG. 18 is a circuit diagram illustrating a circuit constructionof a power conversion apparatus according to a third embodiment;

[0063]FIG. 19 is an equivalent circuit diagram illustrating an operationperformed when an abnormality occurs in an AC power supply in the powerconversion apparatus according to the third embodiment;

[0064]FIG. 20 is a circuit diagram illustrating control of a currentflowing to a reactor according to the third embodiment;

[0065]FIG. 21 is a drawing illustrating a time chart of control of eachswitch means according to the third embodiment;

[0066]FIG. 22 is a drawing illustrating another time chart of thecontrol of each switch means according to the third embodiment;

[0067]FIG. 23 is a flowchart illustrating an example of the control ofeach switch means according to the third embodiment;

[0068]FIG. 24 is a circuit diagram illustrating a conventional powerconversion apparatus;

[0069]FIG. 25 is a circuit diagram illustrating a main circuitconstruction of a power conversion apparatus in the case of asingle-phase three-wire system AC power supply;

[0070]FIG. 26 is a circuit diagram in the case where a conventionaltechnique, with which it is possible to cope with unbalanced loads whenan abnormality occurs in an AC power supply shown in FIG. 24, is appliedto the power conversion apparatus shown in FIG. 25 having thesingle-phase three-wire system AC power supply;

[0071]FIG. 27 is a circuit diagram in the case where a filter capacitoris connected to the conventional power conversion apparatus shown inFIG. 24; and

[0072]FIG. 28 is an explanatory drawing illustrating an operation of thepower conversion apparatus shown in FIG. 27.

BEST MODE FOR CARRYING OUT THE INVENTION

[0073] (First Embodiment)

[0074] Embodiments of the present invention will be described below withreference to the drawings. FIG. 1 is a circuit diagram showing a powerconversion apparatus according to a first embodiment of the presentinvention. In this drawing, reference numerals 112 and 113 denote firstand second AC power supplies (hereinafter simply referred to as “ACpower supplies” in some cases), numerals 116 and 117 capacitors,numerals 120 and 121 resistors (loads) functioning as DC loads, numeral124 a battery, numerals 200 to 203 transistors, numerals 204 to 207diodes, numerals 208 and 209 first and second reactors (hereinaftersimply referred to as “reactors” in some cases), and numerals 210 and211 first and second switch means (hereinafter simply referred to as“switch means” in some cases), which are, for instance, constructed frommechanical relays and the like. Also, although not illustrated, acontrol apparatus is connected to each of the switch means 210 and 211and the transistors 200 to 203, and there are controlled the switchingof the connection state of each of the switch means 210 and 211 and theturning on/off of each of the transistors 200 to 203. The control of theturning on/off of each of the transistors 200 to 203 is performed usinga method that is, for instance, the same as the method shown in FIG. 1of JP Patent No. 2765372. Also, the control of the switching of theconnection state of each of the switch means 210 and 211 is, forinstance, performed by a signal from a microcomputer or the like, adriving circuit that drives the mechanical relays, and the like.

[0075] Switching means S1 is constructed from the transistor 200 and thediode 204, switching means S2 is constructed from the transistor 201 andthe diode 205, switching means S3 is constructed from the transistor 202and the diode 206, and switching means S4 is constructed from thetransistor 203 and the diode 207. The series body of a first switchingmeans is constructed by connecting the switching means S1 including thetransistor 200 and the diode 204 to the switching means S2 including thetransistor 201 and the diode 205 in series, while the series body of asecond switching means is constructed by connecting the switching meansS3 including the transistor 202 and the diode 206 to the switching meansS4 including the transistor 203 and the diode 207 in series.

[0076] Also, a first AC/DC conversion means is constructed by connectingthe first AC power supply 112, the first reactor 208, and the seriesbody of the first switching means in series, while a second AC/DCconversion means is constructed by connecting the second AC power supply113, the second reactor 209, and the series body of the second switchingmeans in series.

[0077] The two capacitors 116 and 117 are connected in series and theinterconnection point C between these capacitors is connected to one endof each of the two AC power supplies 112 and 113, and energy is suppliedby DC voltages obtained by the first and second AC/DC conversion means.

[0078] Also, the resistors 120 and 121 functioning as DC loads arerespectively connected to the two capacitors 116 and 117, and thebattery 124 is connected to the series connection body of the twocapacitors 116 and 117.

[0079] The first switch means 210 is connected between the first ACpower supply 112 and the first reactor 208. In the case where the firstand second AC power supplies 112 and 113 operate normally, the firstswitch means 210 is connected to the contact point A and connects thefirst reactor 208 to the first AC power supply 112. In the case wherethe first or second AC power supply 112 or 113 operates abnormally, thefirst switch means 210 is connected to the contact point B and changesits connection state so that the first reactor 208 is connected to aconnection point between the second reactor 209 and the series body ofthe second switching means.

[0080] The second switch means 211 is connected between the second ACpower supply 113 and the second reactor 209. In the case where the firstand second AC power supplies 112 and 113 operate normally, the secondswitch means 211 is connected to the contact point A and connects thesecond reactor 209 to the second AC power supply 113. In the case wherethe first or second AC power supply 112 or 113 operate abnormally, thesecond switch means 211 is connected to the contact point B and changesits connection state so that the second reactor 209 is connected to aninterconnection point between the capacitors 116 and 117. As a result,the first reactor 208 and the second reactor 209 are connected inseries.

[0081] Next, an operation of this power conversion apparatus will bedescribed. In the case where the AC power supplies 112 and 113 operatenormally, the switch means 210 and 211 are connected to the contactpoints A by the control apparatus, and currents flowing to the reactors208 and 209 are controlled by the control apparatus that alternatelyturns on/off the transistors 200 and 201 and alternately turns on/offthe transistors 202 and 203, so that the power factors of currentsflowing to the AC power supplies 112 and 113 become one. In this manner,AC/DC conversion is performed and the capacitors 116 and 117 arecharged. Also, the charged energy is supplied to the resistors 120 and121.

[0082] When an abnormality, such as a power failure or a momentaryvoltage drop, occurs in either of the AC power supplies 112 and 113, theswitch means 210 and 211 are connected to the contact points B by thecontrol apparatus and energy is supplied from the battery 124 to thecapacitors 116 and 117.

[0083] In the case where the resistors 120 and 121 have the sameresistance value, that is, in the case where these resistors consume thesame amount of power, the energy supplied from the capacitor 116 to theresistor 120 becomes the same as the energy supplied from the capacitor117 to the resistor 121. As a result, the voltages of the capacitors 120and 121 become equal to each other at all times. That is, the voltagevalues of the capacitors 116 and 117 are balanced by the energy suppliedby the battery 124.

[0084] However, in the case of unbalanced loads where the resistors 120and 121 have different values, the voltage values of the capacitors 116and 117 are unbalanced. This is because only the battery 124 performsthe charging of the two capacitors 116 and 117 and it is impossible tocontrol the potential at an interconnection point C between thecapacitors 116 and 117.

[0085] Even if the AC power supplies 112 and 113 operate normally, inthe case of unbalanced loads, the voltage values of the capacitors 116and 117 tend to be unbalanced. However, for instance, by changing theratio between on/off times during the switching of the transistors 200and 201 and the switching of the transistors 202 and 203 in accordancewith the system disclosed in FIGS. 8, 9, and 10 of JP Laid-Open No.2000-278954, such unbalance is eliminated.

[0086] When an abnormality, such as a power failure or a momentaryvoltage drop, occurs in the AC power supply 112 or 113, there occurs aproblem concerning unbalance. Therefore, to solve this problem, theswitch means 210 and 211 are connected to the contact points B and thetransistors 202 and 203 are turned off.

[0087] An equivalent circuit during this operation is shown in FIG. 2.The reactor 208 and the reactor 209 are connected to each other inseries and the control of currents of these reactors 208 and 209 isperformed by the turning on/off of the transistors 200 and 201.

[0088] An operation of the power conversion apparatus at this time willbe described in detail. In FIG. 2, if the value of the resistor 120 isgreater than the value of the resistor 121, both of the capacitors 116and 117 are charged by the battery 124, so that the voltage of thecapacitor 117 attempts to drop and the voltage of the capacitor 116attempts to rise. To suppress this phenomenon, the transistor 200 isfirst turned on (the transistor 201 is turned off), thereby allowing acurrent to flow in the manner shown in FIG. 3 and accumulating theenergy of the capacitor 116 in the reactors 208 and 209.

[0089] Next, the transistor 200 is turned off (the transistor 201 isturned off), thereby allowing a current to flow in the manner shown inFIG. 4 and accumulating, in the capacitor 117, the energy accumulated inthe reactors 208 and 209. By transferring the energy of the capacitor116 to the capacitor 117 in this manner, the voltage of the capacitor116 and the voltage of the capacitor 117 are balanced.

[0090] Current waveforms during this operation are shown in FIG. 5. FIG.5(a) shows a current waveform of the reactor 208, with this currentwaveform being the same as the current waveform of the reactor 209. Notethat the broken line represents the case shown in FIG. 26. FIG. 5(b)shows a gate signal waveform of the transistor 200, FIG. 5(c) shows acurrent waveform flowing to the transistor 200, and FIG. 5(d) shows acurrent waveform flowing to the diode 205. As to the direction of acurrent flowing to each of the reactors 208 and 209, in the case where aflow direction from the left to the right in FIG. 2 is regarded aspositive, the flowing current is negative.

[0091] By turning on/off the transistor 200 in this manner, currentcontrol is performed in the manner shown in FIG. 5A and the currents ofthe reactors 208 and 209 are controlled so that the voltages of the twocapacitors 116 and 117 are balanced.

[0092] Also, during this operation, the two reactors 208 and 209 areconnected in series. Consequently, as to the voltages applied to thesereactors 208 and 209, if the value of the reactor 208 is the same as thevalue of the reactor 209, voltages that are halves of the voltages VC1and VC2 of the capacitors 116 and 117 are applied to the reactors 208and 209, respectively. This means that the voltage applied to each ofthe reactors 208 and 209 becomes half of a capacitor voltage.Accordingly, as shown in FIG. 5(a), the magnitudes of ripple currentsflowing to the reactors 208 and 209 are halved in comparison with thecase shown in FIG. 26.

[0093] Also, in FIG. 2, the battery 124 charges both of the capacitors116 and 117, so that if the value of the resistor 120 is smaller thanthe value of the resistor 121, the voltage of the capacitor 117 attemptsto rise and the voltage of the capacitor 116 attempts to drop. Tosuppress this phenomenon, the transistor 201 is first turned on (thetransistor 200 is turned off), thereby allowing a current to flow in themanner shown in FIG. 6 and accumulating the energy of the capacitor 117in the reactors 208 and 209.

[0094] Next, the transistor 201 is turned off (the transistor 200 isturned off), thereby allowing a current to flow in the manner shown inFIG. 7 and accumulating, in the capacitor 116, the energy accumulated inthe reactors 208 and 209. By transferring the energy of the capacitor117 to the capacitor 116 in this manner, the voltage of the capacitor116 and the voltage of the capacitor 117 are balanced.

[0095] Current waveforms during this operation are shown in FIG. 8. FIG.8(a) shows a current waveform of the reactor 208, with this currentwaveform being the same as the current waveform of the reactor 209. FIG.8(b) shows a gate signal waveform of the transistor 201, FIG. 8(c) showsa current waveform flowing to the transistor 201, and FIG. 8(d) shows acurrent waveform flowing to the diode 204. As to the direction of acurrent flowing to each of the reactors 208 and 209, a flow directionfrom the left to the right is regarded as positive in FIG. 2, so thatthe flowing current is positive.

[0096] By turning on/off the transistor 201 in this manner, currentcontrol is performed in the manner shown in FIG. 8A and the currents ofthe reactors 208 and 209 are controlled so that the voltages of the twocapacitors 116 and 117 are balanced.

[0097] Also, during this operation, the two reactors 208 and 209 areconnected in series. Consequently, as to the voltages applied to thesereactors 208 and 209, if the reactors 208 and 209 have the same value,voltages that are halves of the voltages VC1 and VC2 of the capacitors116 and 117 are applied to the reactors 208 and 209, respectively. Thismeans that the voltage applied to each of the reactors 208 and 209becomes half of a capacitor voltage. Accordingly, the magnitudes ofripple currents flowing to the reactors 208 and 209 are halved incomparison with the case shown in FIG. 26.

[0098] Also, switching is performed only for the transistor 200 whilesetting the transistor 201 turned off in FIG. 5, whereas switching isperformed only for the transistor 201 while setting the transistor 200turned off in FIG. 8. However, in either case, the transistors 200 and201 may be alternatively turned on/off. That is, the same effect isachieved even if the transistor 201 is turned off when the transistor200 is turned on and the transistor 200 is turned off when thetransistor 201 is turned on.

[0099] Next, FIG. 9 shows an example of a control circuit (controlapparatus) that controls the voltage difference between the twocapacitors 116 and 117 by controlling the current of the reactor 208.The construction shown in FIG. 9 is, for instance, the same as theconstruction of a control circuit shown in FIG. 15 of JP Laid-Open No.2000-278954. In this drawing, reference numerals 250 and 251 denotessubtracters, numeral 252 a voltage controller, numeral 253 a currentcontroller, numeral 254 a comparator, numeral 255 a NOT circuit, andnumeral 256 a triangular wave. The voltages VC1 and VC2 of thecapacitors 116 and 117 are inputted into the subtracter 250, while acurrent value i208 of a reactor 208 is inputted into the subtracter 251.

[0100] In a circuit constructed in this manner, first, the voltagedifference between the voltages VC1 and VC2 of the capacitors 116 and117 is detected by the subtracter 250 and is inputted into the voltagecontroller 252. On the basis of the inputted potential difference, thevoltage controller 252 outputs an instruction concerning the currenti208 that should flow to the reactor 208. Next, to have the detectedcurrent value i208 of the reactor 208 follow the current instructiondescribed above, the current difference detected by the subtracter 251is inputted into the current controller 253. The current controller 253outputs an instruction concerning voltages to be applied to the reactors208 and 209, the comparator 254 compares the voltage instruction withthe triangular wave 256 that is a carrier signal, and a result outputtedfrom the comparator 254 becomes an ON-signal of the transistor 200 andbecomes an ON-signal of the transistor 201 via the NOT circuit 255. Withthe aforementioned construction of the control circuit, the voltage ofthe capacitor 116 and the voltage of the capacitor 117 are balanced.

[0101] As described above, with the technique of this embodiment, in thecase where the AC power supply 112 or 113 operates abnormally, even ifthe loads 120 and 121 that are respectively connected to the capacitors116 and 117 are not balanced, it is possible to balance the voltages ofthe two capacitors 116 and 117 by halving the voltages applied to thereactors 208 and 209. This makes it possible to obtain a stable AC/DCconversion operation and also to obtain a power conversion apparatuswhere the ripples of currents of the reactors 208 and 209 caused by theswitching of the transistors 200 and 201 (opening/closing of the firstswitching means) are reduced to one-half and there are reduced lossesand noises.

[0102] It should be noted here that there has been described a modewhere the switching means S1 to S4 are constructed using transistors.However, needless to say, it is possible to obtain the same apparatuseven if semiconductors, such as MOSFET, IGBT, and GTO, are used in placeof the transistors. The same applies to each embodiment to be describedlater.

[0103] It should be noted here that in FIG. 1, the AC power supplies 112and 113 construct a single-phase three-wire system, although it ispossible to construct the same apparatus even in the case of athree-phase four-wire system, as shown in FIG. 10. That is, referencenumerals 300 to 302 denote AC power supplies, numerals 303 to 308transistors, numerals 309 to 314 diodes, numerals 315 to 317 reactors,numerals 318 to 320 switch means, numerals 321 and 322 capacitors,numerals 323 and 324 resistors functioning as loads, and numeral 325 abattery.

[0104] In the case where the AC power supplies 300 to 302 operatenormally, the switch means 318 to 320 are connected to contact points Aand an unillustrated control apparatus performs control so that thepower factors of currents of the reactors 315 to 317 become one byswitching the transistors 303 to 308. In this manner, the capacitors 321and 322 are charged and the charged energy is supplied to the resistors323 and 324.

[0105] When an abnormality, such as a power failure, occurs in any oneof the AC power supplies 300 to 302, the switch means 318 to 320 areconnected to the contact points B by the unillustrated control apparatusand energy is supplied from the battery 325 to the capacitors 321 and322.

[0106] If the values of the resistors 323 and 324 functioning as loadsdiffer from each other during this operation, voltages of capacitors 321and 322 tend to be unbalanced. However, by turning on/off only thetransistors 303 and 304 while setting the transistors 305 to 308 turnedoff, such unbalance is eliminated with the same method as in the firstembodiment.

[0107] Also, as to the voltages applied to reactors 315 to 317, thesereactors 315 to 317 are connected in series, so that if the threereactors have the same value, voltages that are one-third of thevoltages VC1 and VC2 of the capacitors 321 and 322 are applied. As aresult, the voltage applied to each of the reactors 315, 316, and 317becomes one-third of a capacitor voltage.

[0108] As described above, in the case where abnormality occurs in anyone of the AC power supplies 300 to 302, even if the loads 323 and 324that are respectively connected to the two capacitors 321 and 322 arenot balanced, it is possible to balance the voltages of the twocapacitors 321 and 322 by reducing voltages applied to respectivereactors 315, 316, and 317 to one-third. This makes it possible toobtain a stable AC/DC conversion operation and also to obtain anapparatus where the ripples of currents of the reactors 315, 316, and317 caused by the switching of the transistors 303 and 304 are reducedto one-third and there are reduced losses and noises.

[0109] Also, there has been shown the case of the three-phase four-wiresystem in FIG. 10, although it is possible to obtain the sameconstruction and to achieve the same effect even in the case of ann-phase n+1-wire system (n≧4).

[0110] (Second Embodiment)

[0111]FIG. 11 is a circuit diagram showing a power conversion apparatusaccording to the second embodiment of the present invention. In thisdrawing, reference numerals 401, 402, 403, 404, and 405 denote first,second, third, fourth, and fifth switch means (hereinafter simplyreferred to as “switch means” in some cases) that are, for instance,constructed from mechanical relays and the like, and numerals 406 and407 respectively represent first and second filter capacitors(hereinafter simply referred to as “filter capacitors” in some cases)for absorbing ripple currents due to the switching of transistors 200and 201 and the switching of transistors 202 and 203. Also, although notillustrated, a control apparatus is connected to each of the switchmeans 401 to 405 and the transistors 200 to 203, with the controlapparatus controlling the opening/closing of each of the switch means401 to 405 and the turning on/off of each of the transistors 200 to 203.

[0112] The first and second switch means 401 and 402 are respectivelyconnected between a first AC power supply 112 and a first reactor 208and between a second AC power supply 113 and a second reactor 209. Thethird switch means 403 is connected between a connection point betweenthe first switch means 401 and the first reactor 208, and a connectionpoint between the second reactor 209 and the series body of a secondswitching means.

[0113] Also, the fourth switch means 404 is connected between aconnection point between the second switch means 402 and the secondreactor 209, and an interconnection point between the capacitors 116 and117.

[0114] Also, a series body of the fifth switch means 405 and the firstfilter capacitor 406 is connected between a connection point between thethird switch mans 403 and the first reactor 208, and an interconnectionpoint between the capacitors 116 and 117, and the second filtercapacitor 407 is connected between a connection point between the fourthswitch means 404 and the second reactor 209, and an interconnectionpoint between the capacitors 116 and 117.

[0115] Next, an operation of this power conversion apparatus will bedescribed. In the case where the AC power supplies 112 and 113 operatenormally, the switch means 401, 402, and 405 are turned on and theswitch means 403 and 404 are turned off by the control apparatus. Likein the case of the first embodiment, currents of the reactors 208 and209 are controlled so that the power factors of currents flowing to theAC power supplies 112 and 113 become one, and AC/DC conversion isperformed. In this manner, capacitors 116 and 117 are charged and thefilter capacitors 406 and 407 respectively absorb high-frequencycurrents flowing to the reactors 208 and 209.

[0116] When an abnormality, such as a power failure or a momentaryvoltage drop, occurs in either of the AC power supplies 112 and 113, theswitch means 401 and 402 are first turned off by the control apparatus.The switch means 405 is turned off after control to be described lateris performed, and the switch means 403 and 404 are turned on aftercontrol to be described later is performed.

[0117]FIG. 12 shows the final equivalent circuit in the case where anabnormality occurs in the AC power supply 112 or 113. The filtercapacitor 406 is disconnected by the switch means 405 and the reactors208 and 209 are connected in series. In this manner, like in the case ofthe first embodiment, there is suppressed unbalance between the voltagesof the capacitors 116 and 117 even in the case of unbalanced loads.

[0118] If a residual voltage exists in the filter capacitor 407 when theswitch means 404 is turned on, a steep current flows to the switch means404 and the switch means 404 is damaged. A means for avoiding thissituation will be described below.

[0119]FIG. 13 shows an equivalent circuit under a condition before theswitch means 404 and 403 are turned on. To set the voltage V of thefilter capacitor 407 to zero, in the control circuit shown in FIG. 14,proportional control using a gain K is performed to set the current i209flowing to the reactor 209 to zero. The switching of the transistors 202and 203 is controlled to follow a voltage instruction V* that is anoutput of the proportional control. As a result, the voltage V describedabove moves so as to become V*.

[0120] As shown in FIG. 14, an instruction value of i209 is zero under acondition where such control is performed, so that the average value ofi209 becomes zero. Accordingly, V* also becomes zero and V also becomeszero. Because i209 is zero, the average value of a voltage in thereactor 209 also becomes zero. Consequently, both of the voltages atboth ends of the filter capacitor 407 become zero and the average valueof the voltage of the filter capacitor 407 becomes zero.

[0121] Also, as to the turning off of the switch means 405, with aconstruction that is the same as the control circuit shown in FIG. 13,the switch means 405 is turned off under a condition where the currenti208 flowing to the reactor 208 is set to zero.

[0122]FIG. 15 is a time chart concerning the control described above.FIG. 15(a) shows a voltage waveform of the capacitor 407, FIG. 15(b)shows a voltage waveform of the capacitor 406, FIG. 15(c) shows acurrent waveform of the reactor 208, FIG. 15(d) shows a current waveformof the reactor 209, and FIG. 15(e) shows an operation of the switchmeans. The waveforms in these drawings show only average values andthere is omitted ripple currents due to the switching of thetransistors. In these drawings, there is shown a case where currentsremain in the reactors 208 and 209 when an abnormality occurs in the ACpower supply 112 or 113. As a result of the control shown in FIG. 14, aninitial voltage V1 of the filter capacitor 407 becomes zero. Also, in alike manner, the current i208 flowing to the reactor 208 becomes zero.Following this, the switch means 404 and 403 are turned on and theswitching means 405 is turned off. Accordingly, no steep current isgenerated by the discharging of the filter capacitor 407 when the switchmeans 404 is turned on, which prevents a situation where the switchmeans 404 is damaged.

[0123] It should be noted here that in FIG. 15, at an arbitrary timeafter both of the voltages of the filter capacitors 406 and 407 becomezero, the switch means 403 to 405 are switched at the same time.However, as shown in FIG. 16, the switch means 403 to 405 may beindividually switched. FIG. 16(a) shows a voltage waveform of thecapacitor 407, FIG. 16(b) shows a voltage waveform of the capacitor 406,FIG. 16(c) shows a current waveform of the reactor 208, FIG. 16(d) showsa current waveform of the reactor 209, and FIGS. 16(e) to 16(g) eachshow an operation of the switch means. The waveforms in these drawingsshow only average values and there are omitted ripple currents due tothe switching of the transistors. That is, there is obtained the sameeffect even if the switch means 405 is switched after the current of thereactor 208 becomes zero, the switch means 404 is switched after thevoltage of the filter capacitor 407 becomes zero, and the switch means403 is switched after both of the current of the reactor 208 and thevoltage of the filter capacitor 407 become zero.

[0124]FIG. 17 is a flowchart obtained by summarizing flowchartsconcerning control of each switch means. For instance, when the controlapparatus constructed from a microcomputer and the like detects that anabnormality occurs in the AC power supply (step ST1), the apparatusturns off the switch means 401 and 402 (step ST2). Next, when detectingthat the voltage of the capacitor 407 becomes zero as a result of theaforementioned control (step ST3), the control apparatus turns on theswitch means 404 (step ST4). Also, when detecting that the current ofthe reactor 208 becomes zero as a result of the aforementioned control(step ST5), the control apparatus turns off the switch means 405 (stepST6). Further, when detecting that the voltage of the capacitor 407becomes zero (step ST7), the control apparatus turns on the switch means403 (step ST8).

[0125] It should be noted here that in this embodiment, there has beendescribed a case where a current remains in the reactor 208 when anabnormality occurs in the AC power supply 112 or 113. However, in thecase where no current remains therein, the switch means 405 may beturned off concurrently with the occurrence of the abnormality of the ACpower supply 112 or 113.

[0126] As described above, with the technique of this embodiment, evenin the case where there are connected the filter capacitors 406 and 407for absorbing ripple currents, a steep current is not caused by asituation where electric charges remaining in the filter capacitor 407are shirt-circuited by the switch means 404. As a result, it is possibleto circumvent a problem that the switch means 404 is damaged. Also, theswitch means 405 is turned off by setting the current of the reactor 208to zero, so that there is circumvented a problem that the switch means405 is damaged by energy accumulated in the reactor 208 when the switchmeans 405 is turned off.

[0127] Also, it is possible to balance the voltages of the twocapacitors 116 and 117 by halving the voltages applied to the reactors208 and 209. Consequently, it becomes possible to obtain a stable AC/DCconversion operation and also to obtain a power conversion apparatuswhere the ripples of currents of the reactors 208 and 209 caused by theswitching of the transistors 200 and 201 are reduced to one-half andthere are reduced losses and noises.

[0128] It should be noted here that in this embodiment, there has beendescribed the single-phase three-wire system as an example. However,needless to say, it is possible to obtain the same construction and toachieve the same effect even in the case of a three-phase four-wiresystem shown in FIG. 10 or an n-phase n+1-wire system (n≧4).

[0129] (Third Embodiment)

[0130]FIG. 18 is a circuit diagram showing a power conversion apparatusaccording to the third embodiment of the present invention. In thisembodiment, to realize cost reduction, the fifth switch means 405 of thesecond embodiment is omitted and only the first filter capacitor 406 isconnected between the connection point between the third switch means403 and the first reactor 208, and the interconnection point between thecapacitors 116 and 117. Other constructions are the same as those in thesecond embodiment.

[0131] Next, an operation of this power conversion apparatus will bedescribed. In the case where the AC power supplies 112 and 113 operatenormally, the switch means 401 and 402 are turned on and the switchmeans 404 and 403 are turned off. Like in the case of the secondembodiment shown in FIG. 11, AC/DC conversion is performed bycontrolling the currents of the reactors 208 and 209 so that the powerfactors of the currents flowing to the AC power supplies 112 and 113become one. In this manner, the capacitors 116 and 117 are charged andthe filter capacitors 406 and 407 respectively absorb high-frequencycurrents flowing to the reactors 208 and 209.

[0132] When an abnormality, such as a power failure or a momentaryvoltage drop, occurs in either of the AC power supplies 112 and 113, theswitch means 401 and 402 are first turned off by a control apparatus andthe switch means 404 and 403 are turned on after control to be describedlater is performed.

[0133] The final equivalent circuit in the case where an abnormalityoccurs in the AC power supply 112 or 113 is shown in FIG. 19. A parallelconnection body of the reactor 209 and the filter capacitor 406 isconnected to the reactor 208 in series, which means that an impedance isconnected to the reactor 208 in series. As a result, the ripple currentflowing to the reactor 208 is reduced in comparison with the case shownin FIG. 26.

[0134] By the way, in FIG. 18, if a residual voltage exists in thefilter capacitor 407 when the switch means 404 is turned on, a steepcurrent flows to the switch means 404 and this switch means 404 isdamaged. A means for circumventing this situation is the same as thatdescribed in the second embodiment and therefore is not described inthis embodiment. Also, if a residual voltage exists in the filtercapacitor 406 when the switch means 403 is turned on, unnecessaryresonance is generated between the reactor 209 and the filter capacitor406 in FIG. 19, which results in energy losses. A means forcircumventing this problem will be described below.

[0135] In FIG. 20, before the switch means 403 is turned on, thetransistors 200 and 201 are controlled by a control circuit that is thesame as that shown in FIG. 14 so that the current i208 flowing to thereactor 208 becomes zero. Accordingly, as a result of an operation thatis the same as that in the second embodiment, the voltage of the filtercapacitor 406 becomes zero.

[0136] FIGS. 21(a) to (e) show voltages of the filter capacitors 406 and407, waveforms of i208 and i209, and an operation of the switch means.FIG. 21(a) shows a voltage waveform of the capacitor 407, FIG. 21(b)shows a voltage waveform of the capacitor 406, FIG. 21(c) shows acurrent waveform of the reactor 208, FIG. 21(d) shows a current waveformof the reactor 209, and FIG. 21(e) shows the operation of the switchmeans are operated. The waveforms in these drawings show only averagevalues and there are omitted ripple currents caused by the switching ofthe transistors. By a control circuit that is the same as that in FIG.14 described above, the initial values V1 and V2 of the capacitors 406and 407 become zero and the switch means 404 and 403 are turned on,thereby obtaining the construction shown in FIG. 19 and balancing thevoltages of the capacitors 116 and 117.

[0137] It should be noted here that in FIG. 21, at an arbitrary timeafter both of the voltages of the filter capacitors 406 and 407 becomezero, the switch means 403 and 404 are switched at the same time.However, as shown in FIG. 22, the switch means 403 and 404 may beindividually switched. That is, there is obtained the same effect evenif the switch means 403 is switched after the voltage of the filtercapacitor 406 becomes zero and the switch means 404 is switched afterthe voltage of the filter capacitor 407 becomes zero. FIG. 22(a) shows avoltage waveform of the capacitor 407, FIG. 22(b) shows a voltagewaveform of the capacitor 406, FIG. 22(c) shows a current waveform ofthe reactor 208, FIG. 22(d) shows a current waveform of the reactor 209,FIG. 22(e) shows an operation of the switch means 404, and FIG. 22(f)shows an operation of the switch means 403. The waveforms in thesedrawings show only the average values and there are omitted ripplecurrents due to the switching of the transistors.

[0138] When flowcharts concerning control of respective switch meansduring this operation are summarized, there is obtained the flowchartshown in FIG. 23. For instance, when a control apparatus constructedfrom a microcomputer and the like detects that an abnormality occurs inthe AC power supply (step ST11), the apparatus turns off the switchmeans 401 and 402 (step ST12). Next, when detecting that the voltage ofthe capacitor 406 becomes zero as a result of the aforementioned control(step ST13), the control apparatus turns on the switch means 403 (stepST14). Also, when detecting that the voltage of the capacitor 407becomes zero as a result of the aforementioned control (step ST15), thecontrol apparatus turns on the switch means 404 (step ST16).

[0139] As described above, with the technique of this embodiment, in thecase where there are connected the filter capacitors 406 and 407 forabsorbing ripple currents, even if the switch means 405 of the secondembodiment is omitted for cost reduction, a steep current is not causedby a situation where electric charges remaining in the filter capacitor407 are shirt-circuited by the switch means 404. As a result, it ispossible to circumvent a problem that the switch means 404 is damaged.Also, it is possible to prevent the occurrence of unnecessary resonancebetween the filter capacitor 406 and the reactor 209 and there aresuppressed energy losses due to the resonance.

[0140] Also, the parallel connection body of the reactor 209 and thefilter capacitor 406 is connected to the reactor 208 in series, whichmeans that that an impedance is connected to the reactor 208 in series.As a result, it becomes possible to reduce the ripple current flowing tothe reactor 208 in comparison with the case shown in FIG. 26 and toreduce losses and noises.

[0141] Also, it is possible to balance the voltages of the twocapacitors 116 and 117, which makes it possible to obtain a stable AC/DCconversion operation.

[0142] It should be noted here that in this embodiment, the single-phasethree-wire system has been described as an example. However, needless tosay, it is possible to obtain the same construction and to achieve thesame effect even in the case of a three-phase four-wire system shown inFIG. 10 or an n-phase n+1-wire system (n≧4).

INDUSTRIAL APPLICABILITY

[0143] It is possible to use the power conversion apparatus of thepresent invention as a power conversion apparatus, such as anuninterruptible power supply capable of supplying power to a load evenif an abnormality, such as a power failure or a momentary voltage drop,occurs in a system power supply.

1. A power conversion apparatus comprising: a first AC/DC conversionmeans constructed by connecting a first AC power supply, a firstreactor, and a series body of a first switching means in series; asecond AC/DC conversion means constructed by connecting a second ACpower supply, a second reactor, and a series body of a second switchingmeans in series; two capacitors connected in series, an interconnectionpoint of the two capacitors being connected to one end of each of thetwo AC power supplies, and the two capacitors receiving energy suppliedby DC voltages obtained by the first and second AC/DC conversion means;a load that is respectively connected to the two capacitors; and abattery connected to the two capacitors that are connected in series,wherein the power conversion apparatus comprises: a first switch meansthat is connected between the first AC power supply and the firstreactor, connects the first reactor to the first AC power supply if thefirst and second AC power supplies operate normally, and connects thefirst reactor to a connection point between the second reactor and theseries body of the second switching means if one of the first and secondAC power supplies operates abnormally; a second switch means that isconnected between the second AC power supply and the second reactor,connects the second reactor to the second AC power supply if the firstand second AC power supplies operate normally, and connects the firstreactor and the second reactor in series by connecting the secondreactor to an interconnection point between the capacitors if one of thefirst and second AC power supplies operates abnormally; and a controlapparatus that controls a voltage difference between the two capacitors,wherein: if the first and second AC power supplies operate normally, thecontrol apparatus controls a current flowing to the first reactor usingthe series body of the first switching means to perform AC/DC conversionand controls a current flowing to the second reactor using the seriesbody of the second switching means to perform AC/DC conversion; and ifone of the first and second AC power supplies operates abnormally, thecontrol apparatus has the battery supply energy to the two capacitorsand controls the current flowing to the first reactor using the seriesbody of the first switching means.
 2. A power conversion apparatuscomprising: a first AC/DC conversion means constructed by connecting afirst AC power supply, a first reactor, and a series body of a firstswitching means in series; a second AC/DC conversion means constructedby connecting a second AC power supply, a second reactor, and a seriesbody of a second switching means in series; two capacitors connected inseries, an interconnection point of the two capacitors being connectedto one end of each of the two AC power supplies, and the two capacitorsreceiving energy supplied by DC voltages obtained by the first andsecond AC/DC conversion means; a load that is respectively connected tothe two capacitors; and a battery connected to the two capacitors thatare connected in series, wherein the power conversion apparatuscomprises: a first switch means that is connected between the first ACpower supply and the first reactor; a second switch means that isconnected between the second AC power supply and the second reactor; athird switch means that is connected between a connection point betweenthe first switch means and the first reactor, and a connection pointbetween the second reactor and the series body of the second switchingmeans; a fourth switch means that is connected between a connectionpoint between the second switch means and the second reactor, and aninterconnection point between the capacitors; a series body of a fifthswitch means and a first filter capacitor that is connected between aconnection point between the third switch means and the first reactor,and an interconnection point between the capacitors; a second filtercapacitor that is connected between a connection point between thefourth switch means and the second reactor, and an interconnection pointbetween the capacitors; and a control apparatus that controls a voltagedifference between the two capacitors, wherein: if the first and secondAC power supplies operate normally, the control apparatus turns on thefirst switch means, the second switch means, and the fifth switch meansand turns off the third switch means and the fourth switch means, sothat a current flowing to the first reactor is controlled using theseries body of the first switching means to perform AC/DC conversion, ahigh-frequency current flowing to the first reactor is absorbed usingthe first filter capacitor, a current flowing to the second reactor iscontrolled using the series body of the second switching means toperform AC/DC conversion, and a high-frequency current flowing to thesecond reactor is absorbed using the second filter capacitor; and if oneof the first and second AC power supplies operate abnormally, thecontrol apparatus turns off the first switch means and the second switchmeans, sets a voltage of the second filter capacitor to approximatelyzero through switching of the series body of the second switching means,sets a current of the first reactor to approximately zero throughswitching of the series body of the first switching means, turns on thethird switch means and the fourth switch means, turns off the fifthswitch means, connects the first reactor to a connection point betweenthe second reactor and the series body of the second switching means,and connects the first reactor to the second reactor in series, so thatenergy is supplied to the two capacitors using the battery and a currentflowing to the first reactor is controlled using the series body of thefirst switching means.
 3. A power conversion apparatus comprising: afirst AC/DC conversion means constructed by connecting a first AC powersupply, a first reactor, and a series body of a first switching means inseries; a second AC/DC conversion means constructed by connecting asecond AC power supply, a second reactor, and a series body of a secondswitching means in series; two capacitors connected in series, aninterconnection point of the two capacitors being connected to one endof each of the two AC power supplies, and the two capacitors receivingenergy supplied by DC voltages obtained by the first and second AC/DCconversion means; a load that is respectively connected to the twocapacitors; and a battery connected to the two capacitors that areconnected in series, wherein the power conversion apparatus comprises: afirst switch means that is connected between the first AC power supplyand the first reactor; a second switch means that is connected betweenthe second AC power supply and the second reactor; a third switch meansthat is connected between a connection point between the first switchmeans and the first reactor, and a connection point between the secondreactor and the series body of the second switching means; a fourthswitch means that is connected between a connection point between thesecond switch means and the second reactor, and an interconnection pointbetween the capacitors; a first filter capacitor that is connectedbetween a connection point between the third switch means and the firstreactor, and an interconnection point between the capacitors; a secondfilter capacitor that is connected between a connection point betweenthe fourth switch means and the second reactor, and an interconnectionpoint between the capacitors; and a control apparatus that controls avoltage difference between the two capacitors, wherein: if the first andsecond AC power supplies operate normally, the control apparatus turnson the first switch means and the second switch means and turns off thethird switch means and the fourth switch means, so that a currentflowing to the first reactor is controlled using the series body of thefirst switching means to perform AC/DC conversion, a high-frequencycurrent flowing to the first reactor is absorbed using the first filtercapacitor, a current flowing to the second reactor is controlled usingthe series body of the second switching means to perform AC/DCconversion, and a high-frequency current flowing to the second reactoris absorbed using the second filter capacitor; and if one of the firstand second AC power supplies operate abnormally, the control apparatusturns off the first switch means and the second switch means, sets avoltage of the first filter capacitor to approximately zero throughswitching of the series body of the first switching means, sets avoltage of the second filter capacitor to approximately zero throughswitching of the series body of the second switching means, turns on thethird switch means and the fourth switch means, connects the firstreactor to a connection point between the second reactor and the seriesbody of the second switching means, and connects the first reactor to aparallel connection body of the second reactor and the first filtercapacitor in series, so that energy is supplied to the two capacitorsusing the battery and a current flowing to the first reactor iscontrolled using the series body of the first switching means.